1. Field of the Invention
The subject invention concerns fire extinguishing systems and, more particularly, explosion protection systems for use in applications such as aircraft fuel tanks.
2. Description of the Prior Art
In the prior art, both suppression and inerting systems have been developed to provide protection against explosions. Explosion suppression systems are intended to extinguish a combustion condition before uncontrolled combustion achieves unacceptable levels. For example, in a fuel tank an explosion suppression system is intended to extinguish a combustion condition before the pressure rise in the tank causes the tank to explode.
Explosion suppression accepts that a fire already exists with a flame front moving through the vessel. Behind the flame front, heat of combustion is released and local temperatures and pressures are high. The suppressant must absorb sufficient combustion energy to lower temperatures and pressures and to quench the fire. Ahead of the flame front, flame propagation must be arrested to stop the release of additional energy.
Unlike suppression systems, inerting systems are designed to provide a nonexplosive environment in the area of concern. Thus, even in the presence of an ignition source, no explosion will occur because combustion cannot be sustained. For example, in fuel tanks inerting systems typically supplant oxygen in the tank ullage with nitrogen, helium, carbon dioxide, or other inert gas. This condition is maintained for as long as protection is desired by adjusting for expansion of the tank ullage and for variations in external pressure.
Early inerting systems had several disadvantages and deficiencies. For example, some fuel tank inerting systems required that dissolved oxygen be purged from the fuel before it was placed in the tank. Another common disadvantage was that sufficient quantities of the inerting gas were not always conveniently available. To address the problem of dissolved oxygen in the fuel, some subsequent inerting systems used halon as an inerting gas. These systems were an improvement in that they would tolerate the presence of oxygen but they did not overcome the problem of a convenient source of inerting gas.
To overcome the difficulty of inert gas availability, still other inerting systems employed inert gas generation equipment. Typically, these systems used nitrogen as an inerting gas with the generation equipment stripping oxygen from air to provide a nitrogen supply. The disadvantage with these systems was that they tended to be mechanically complicated and, therefore, costly to construct, operate and maintain. Also, these systems produced a limited flow of inert gas that, in many cases, was inadequate to meet peak demands, such as large variants in ullage volume or pressure. Thus, inert gas storage tanks were generally required for these systems, adding significantly to system size and weight. For some applications such as aircraft fuel tanks, such limitations seriously compromised the availability and operational performance of the vehicle.
Despite performance and cost penalties of inerting systems, the prior explosion protection relied on inerting systems because there was no practical alternative. Prior art suppressant systems proposed vacuum tubes and other sensors to detect combustion conditions These sensors were connected to various suppressant storage devices containing freon, carbon tetrachloride, halon or other inert gas. The storage device released the suppressant through explosive impulse or other mechanism in response to a signal from the sensor. Such prior art systems were generally found to be too slow or too unreliable for arresting an explosion after combustion had begun. Other explosion protection systems, such as reticulated foam systems, were too heavy or otherwise unsuitable for many applications.
Nevertheless, a practical explosion suppression system would offer significant advantages over inerting systems. Since suppression systems are mechanically simpler, they would tend to be more reliable than inerting systems. Such simplicity would also tend to make the cost of constructing, installing and maintaining a suppression system substantially lower than corresponding costs for an inerting system. Moreover, the cost and maintenance associated with obtaining and handling consumables used in inerting systems would be eliminated. Other advantages of such mechanical simplicity and the avoidance of consumables include a higher degree of availability and a reduction in system weight. Moreover, a suppression system would be installed on existing vehicles more easily than an inerting system. Accordingly, there was a need in the prior art for an effective explosion suppression system that was reliable, but compact and required limited, if any, maintenance.